Which has maximum ionization potential?
A) Nitrogen.
B) Oxygen.
C) ${O^ + }$
D) $Na$

Hint:We know that Ionization energy is defined as the energy required removing one mole of electrons from one mole of isolated gaseous atoms or ions as the atomic size increases and also increases the ionization energy.

Complete step by step answer:

The ionization energy depends on the atomic number of an element greater the atomic number smaller is the ionization energy.

We know the atomic number of nitrogen is $7$. The electronic configuration of nitrogen is $1{s^2}2{s^2}2{p^3}$. We know that the atomic number of oxygen is eight and the electronic configuration is $1{s^2}2{s^2}2{p^4}$. ${O^ + }$ Ion is formed by losing an electron from its outermost shell and the electronic configuration of ${O^ + }$ is similar to nitrogen. The atomic number of sodium is eleven and the electronic configuration is $1{s^2}2{s^2}2{p^6}3{s^1}$. Among all the elements nitrogen is the smallest of all and has a half-filled configuration so it is harder to remove electrons from nitrogen thus the first ionization energy of nitrogen is greater than the oxygen and sodium and the ionization energy of an oxygen ion is somewhat similar to nitrogen.
Thus nitrogen and oxygen ions are the elements having maximum ionization potential.

Hence option A and C are correct.


Note:
We must remember that the first ionization energy of an atom (or) molecule is the energy required to remove one mole of electrons from one mole of gaseous atoms (or) ions.
Example:
The first ionization energy of the hydrogen molecule is greater than the hydrogen atom. This can be explained based on their electronic configuration.
The ground state configuration of hydrogen molecule is $\sigma$$1s^2$
The ground state configuration of the hydrogen atom is ${\text{1}}{{\text{s}}^{\text{1}}}$.
The molecules which have filled configuration have more ionization energy than the one with half-filled configuration. The Hydrogen molecule has a filled configuration so its first ionization energy is higher than the hydrogen atom.